One Piece. The greatest manga of all time, even though it's still not done. But this is the anime, and we're going to only judge it based on that.
Plot - [9.5/10] - To be honest, I cannot find an anime that can be as deeply rich as One Piece is. Its plot is incredibly well-done, and everything always fits together even after 800+ episodes. I cannot even describe it, it's just pure amazing. But like all plots, there is never a perfect 10/10.
Characters - [9/10] - One Piece has the most unique characters (and most unique recurring characters) out of all anime. Not only that, but each key character has a continuous purpose in the story, and does not get left out that easily. This does include the villains of One Piece, which are by far some of the best. The only reason this doesn't get a 10/10 is because there are usually characters that just don't make much out of what they seem. The full potential is not brought out.
Art & Animation - [6/10] - While not a very artistic anime, it does make up for it by creating unique looks for each character with special voices and/or laughs. The animation is also not that terrible, but after a certain point of episodes it was just heavily dragged down and the end result... disappointing, to say the least. Nothing more to say.
Story - [7.5/10] - I'll say this straight up.The story is really bad. The pacing in the newer episodes is really bad. If you're watching, expecting it to get good after 800+ episodes, it won't. Every new episode is usually 10 minutes plot, the rest being the incredibly long opening and pointless scenes that make up the rest of the episode. At this point I'd rather watch Naruto's fillers again. Only reason it's 7.5 is because the plot is carrying it on its back.
Music and Sounds - [7/10] - While still good, it's not the type of music you'd hear in anime such as AoT or Guilty Crown. The music serves its purpose in One Piece, and that's fine. On the other hand, the sounds in One Piece get repetitive pretty quickly. And you start to get bored. Some sounds are overused to the point where you don't even want to hear it play. You'd rather just mute.
Overall, I'd give it an 8/10. It's my 2nd anime after Naruto Shippuden, and it's still good. Go give it a try. But don't come back disappointed in the recent episodes with the pacing.